The present invention relates, most generally, to high performance optical systems and lithography methods. More particularly, the present invention relates to an apparatus and method for compensating for the effects of intrinsic birefringence in optical systems using cubic crystalline optical elements.
In order to increase levels of device integration for integrated circuit and other semiconductor devices, there is a drive to produce device features having smaller and smaller dimensions. In today""s rapidly advancing semiconductor manufacturing industry, there is a related drive to produce such device features in a reliable and repeatable manner.
Optical lithography systems are commonly used in the fabrication process to form images of device patterns upon semiconductor substrates. The resolving power of such systems is proportional to the exposure wavelength; therefore, it is advantageous to use exposure wavelengths that are as short as possible. For sub-micron lithography, deep ultraviolet light having a wavelength of 248 nanometers or shorter is commonly used. Wavelengths of interest include 193 and 157 nanometers.
At ultraviolet or deep ultraviolet wavelengths, the materials used to form the lenses, windows, and other optical elements of the lithography system, are of critical significance. Such optical elements must be compatible with the short wavelength light used in these lithography systems.
Calcium fluoride and other cubic crystalline materials such as barium fluoride, lithium fluoride, and strontium fluoride, represent some of the materials being developed for use as optical elements for 157 nanometer lithography, for example. These single crystal fluoride materials have a desirably high transmittance compared to ordinary optical glass and can be produced with good homogeneity.
Accordingly, such cubic crystalline materials are useful as optical elements in short wavelength optical systems such as wafer steppers and other projection printers used to produce small features on substrates such as semiconductor and other wafers used in the semiconductor manufacturing industry. In particular, calcium fluoride finds particular advantage in that it is an easily obtained cubic crystalline material and large high purity single crystals can be grown.
A primary concern for the use of cubic crystalline materials for optical elements in deep ultraviolet lithography systems is anisotropy of refractive index inherent in cubic crystalline materials; this is referred to as xe2x80x9cintrinsic birefringence.xe2x80x9d It has been recently reported [J. Burnett, Z. H. Levine, and E. Shipley, xe2x80x9cIntrinsic Birefringence in 157 nm materials,xe2x80x9d Proc. 2nd Intl. Symp on 157 nm Lithography, Austin, Intl SEMATEC, ed. R. Harbison, 2001] that cubic crystalline materials such as calcium fluoride, exhibit intrinsic birefringence that scales as the inverse of the square of the wavelength of light used in the optical system. The magnitude of this birefringence becomes especially significant when the optical wavelength is decreased below 250 nanometers and particularly as it approaches 100 nanometers. Of particular interest is the effect of intrinsic birefringence at the wavelength of 157 nanometers (nm), the wavelength of light produced by an F2 excimer laser favored in the semiconductor manufacturing industry.
Birefringence, or double-refraction, is a property of refractive materials in which the index of refraction is anisotropic. For light propagating through a birefringent material, the refractive index varies as a function of polarization and orientation of the material with respect to the propagation direction. Unpolarized light propagating through a birefringent material will generally separate into two beams with orthogonal polarization states.
When light passes through a unit length of a birefringent material, the difference in refractive index for the two ray paths will result in an optical path difference or retardance. Birefringence is a unitless quantity, although it is common practice in the lithography community to express it in units of nm/cm. Birefringence is a material property, while retardance is an optical delay between polarization states. The retardance for a given ray through an optical system may be expressed in nm, or it may be expressed in terms of number of waves of a particular wavelength.
In uniaxial crystals, such as magnesium fluoride or crystal quartz, the direction through the birefringent material in which the two refracted beams travel with the same velocity is referred to as the birefringence axis. The term optic axis is commonly used interchangeably with birefringence axis when dealing with single crystals. In systems of lens elements, the term optical axis usually refers to the symmetry axis of the lens system. To avoid confusion, the term optical axis will be used hereinafter only to refer to the symmetry axis in a lens system. For directions through the material other than the birefringence axis, the two refracted beams will travel with different velocities. For a given incident ray upon a birefringent medium, the two refracted rays are commonly described as the ordinary and extraordinary rays. The ordinary ray is polarized perpendicular to the birefringence axis and refracts according to Snell""s Law, and the extraordinary ray is polarized perpendicular to the ordinary ray and refracts at an angle that depends on the direction of the birefringence axis relative to the incident ray and the amount of birefringence. In uniaxial crystals, the birefringence axis is oriented along a single direction, and the magnitude of the birefringence is constant throughout the material. Uniaxial crystals are commonly used for optical components such as retardation plates and polarizers.
In contrast, however, cubic crystals have been shown to have both a birefringence axis orientation and magnitude that vary depending on the propagation direction of the light with respect to the orientation of the crystal lattice. In addition to birefringence, which is the difference in the index of refraction seen by the two eigenpolarizations, the average index of refraction also varies as a function of angle of incidence, which produces polarization independent phase errors.
Crystal axis directions and planes are described herein using Miller indices, which are integers with no common factors and that are inversely proportional to the intercepts of the crystal planes along the crystal axes. Lattice planes are given by the Miller indices in parentheses, e.g. (100), and axis directions in the direct lattice are given in square brackets, e.g. [111]. The crystal lattice direction, e.g. [111], may also be referred to as the [111] crystal axis of the material or optical element. The (100), (010), and (001) planes are equivalent in a cubic crystal and are collectively referred to as the {100} planes. For example, light propagating through an exemplary cubic crystalline optical element along the [110] crystal axis experiences the maximum birefringence, while light propagating along the [100] crystal axis experiences no birefringence.
Thus, as a wavefront propagates through an optical element constructed from a cubic crystalline material, the wavefront may be retarded because of the intrinsic birefringence of the optical element. The retardance magnitude and orientation may each vary, because the local propagation angle through the material varies across the wavefront. Such variations may be referred to as xe2x80x9cretardance aberrations.xe2x80x9d Retardance aberrations split a uniformly polarized wavefront into two wavefronts with orthogonal polarizations. Each of the orthogonal wavefronts will experience a different refractive index, resulting in different wavefront aberrations. These aberrations are capable of significantly degrading image resolution and introducing distortion of the image field at the wavelengths of interest, such as 157 nm, particularly for sub-micron projection lithography in semiconductor manufacturing. It can be therefore seen that there is a need in the art to compensate for wavefront aberrations caused by intrinsic birefringence of cubic crystalline optical elements, which can cause degradation of image resolution and image field distortion, particularly in projection lithography systems using light having wavelengths in the deep ultraviolet range.
To address these and other needs, and in view of its purposes, the present invention provides a method and apparatus for preventing intrinsic birefringence in cubic crystalline optical systems from causing wavefront aberrations. The crystal axes of the cubic crystalline lens elements are oriented to minimize net retardance by balancing the retardance contributions from the individual lens elements.
In one exemplary embodiment, the present invention provides an optical system which includes a projection lens formed of a plurality of optical elements, two or more of which are constructed from cubic crystalline material and oriented with their [110] cubic crystalline lattice direction along the system optical axis and with relative rotations about the optical axis to give reduced retardance for light propagating at small angles relative to the system optical axis, and one or more elements oriented with the optical axis substantially along the [100] cubic crystalline lattice direction to give reduced retardance for off-axis light propagating at larger angles with respect to the system optical axis.
In another exemplary embodiment, the present invention provides an optical system which includes four optical elements which are constructed from cubic crystalline material and oriented with the optical axis substantially along their [110] cubic crystalline lattice directions. The optical elements are oriented about the optical axis to give reduced retardance for light propagating at small angles relative to the system optical axis. The system further includes an optical element oriented with its [100] crystal lattice direction substantially along the optical axis to give reduced retardance for light propagating at larger angles with respect to the system optical axis.
In another exemplary embodiment, the present invention provides an optical system that includes a plurality of optical elements, two or more of which are constructed from cubic crystalline material and oriented with their [110] cubic crystalline lattice direction along the optical axis of the system, and with relative rotations about the optical axis to give reduced retardance for light propagating at small angles relative to the [110] lattice direction. A stress-induced birefringence is applied to either a [110] cubic crystal optical element or a further optical element such as a non-cubic crystalline element or a [100] optical element, to reduce residual retardance of the optical system.
In another exemplary embodiment, the present invention provides a method and apparatus for reducing retardance aberrations caused by intrinsic birefringence by providing a lens system, orienting two or more elements with the optical axis substantially along the [110] cubic crystalline lattice directions of the elements and one or more elements with the optical axis substantially along the [100] cubic crystalline lattice directions of the elements, and providing optimized relative rotations of the elements about the optical axis.
In another exemplary embodiment, the present invention provides a method and apparatus for reducing retardance aberrations caused by intrinsic birefringence by providing a lens system defined by a lens prescription, then splitting at least one of the elements of the lens system into multiple cubic crystalline components, oriented to reduce retardance aberrations while maintaining the overall element dimensions defined by the lens prescription.
In yet another exemplary embodiment, the present invention provides a method and apparatus for reducing retardance caused by intrinsic birefringence by providing a lens system with at least two cubic crystalline optical elements and providing a stress-induced birefringence to at least one of the optical elements to reduce residual retardance variations.
Another aspect of the present invention is an apparatus and method for compensating for residual astigmatism due to variations in the average index of refraction in the cubic crystalline optical elements, through the use of at least one optical element whose base radius of curvature differs in orthogonal directions.
In another exemplary embodiment, the present invention provides a photolithography tool including one of the above-described optical systems.
In another exemplary embodiment, the present invention provides a method and apparatus for using the selectively oriented crystalline lens elements to form semiconductor devices on semiconductor substrates used in the semiconductor manufacturing industry.
In another exemplary embodiment, the present invention provides a semiconductor device formed using a lithography tool including the selectively oriented cubic crystalline lens elements.